Bump underscore from 1.13.4 to 1.13.6 (#1421)

Bumps [underscore](https://github.com/jashkenas/underscore) from 1.13.4 to 1.13.6.
- [Release notes](https://github.com/jashkenas/underscore/releases)
- [Commits](jashkenas/underscore@1.13.4...1.13.6)

---
updated-dependencies:
- dependency-name: underscore
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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package.json

@@ -97,7 +97,7 @@
         "punycode": "2.1.1",
         "seedrandom": "^3.0.5",
         "tldts": "^5.7.91",
-        "underscore": "1.13.4",
+        "underscore": "1.13.6",
         "web-ext": "^7.2.0"
     },
     "babel": {